Professor Forbes’s Ninth Letter on Glaciers. 335 
slaty cleavage of the ice, in the way that I have explained in 
my Seventh Letter. Inaglacier then, whose slope is nearly 
constant and small, I should expect a condensation of the ice 
longitudinally, and a swelling of the surface depending upon 
the motion of the plastic ice in the direction of least resist- 
ance. Now this is exactly what we have in the results 
of measurement. If the annexed figure represent the plan 
Lal/ aan LULU YU 
Uo an WA 
of the glacier, and the ice be divided into imaginary com- 
partments by vertical sections; since, whilst AB moves 
177 feet, CD moves but 141 feet, there is a condensation 
of the mass of ice ABCD, from back to front, of no less 
than 36 feet in that time, and so for the successive slices 
EF, &c. How, then, is this shrinking to be accounted 
for? Not by the mere internal melting, for that would pro- 
duce merely a lowering of the surface, and a subsidence of 
the level of the ice ; such as I have shewn* actually takes 
place in other glaciers whose sections move with increasing 
velocity on the whole. There is only a vis @ tergo which 
can approximate the sections together, and, as we read in 
the Comptes Rendus, squeeze the moraine longitudinally, giv- 
ing it a greater breadth,t and coudense the entire body of 
the ice so as to make it more compact in texture.t 
If we take a vertical section instead of a plan (see next 
page), the slice abcd must be condensed into the higher 
and shorter solid cdef, and so of the rest, and the surface 
— 
* Travels, p. 153. 
- t Les “ Moraines médianes s’élargissent dans la méme proportion que 
le mouvement se ralentit.” Comptes Rendus, 9th Dec. 1844, p- 1301. 
t Ib. p. 1306, line 29. 
